In recent years, cross roller bearings have been incorporated in turning portions and swinging portions of industrial robots, machine tools, medical instruments, optical instruments, etc. Cross roller bearings of various types and sizes have been commercialized for various applications. Particularly, thin-walled cross roller bearings have contributed to reduction in size and weight of machinery.
For example, an ultrathin-type cross roller bearing disclosed by the applicant of the present application is used in industrial robots, optical instruments, medical instruments, etc. The ultrathin-type cross roller bearing can reduce overall size and weight through impartment of an ultrathin wall thickness to an outer ring and an inner ring. The ultrathin-type cross roller bearing employs circular columnar separators disposed between rollers so as to maintain the rollers in an appropriate posture, reduces the size and weight of the bearing to the greatest possible extent, and ensures reliable lubrication. The ultrathin-type cross roller bearing comprises the outer ring, the inner ring, the rollers intervening between the outer ring and the inner ring, and the separators disposed between the rollers (see, for example, Japanese Patent Application Laid-Open No. 2009-287587.)
Also known is a hollow actuator used in joints of industrial robots, etc. A rolling bearing provided in the known hollow actuator employs an inner ring and an outer ring, both of which have threaded holes formed therein. An output plate and an ultrasonic motor are attached to the rolling bearing such that the ultrasonic motor is attached to one side of the outer ring, while the output plate is attached to the other side of the inner ring, thereby preventing entry into the rolling bearing of abrasion powder generated by the ultrasonic motor (see, for example, Japanese Patent Application Laid-Open No. 2016-19447).
There is known a cross roller bearing for rotatably supporting rotation shafts of various instruments and devices. The cross roller bearing includes an inner ring and a two-piece outer ring. The cross roller bearing is configured as follows: in order to facilitate positioning of the two pieces of the outer ring, a tongue-and-groove joint is employed for joining the two pieces together, and the two pieces of the outer ring are fastened together by use of bolts with rollers intervening between the inner ring and the outer ring (see, for example, Japanese Utility Model Application Laid-Open No. H04-78328).
Incidentally, since the above-mentioned ultrathin-type cross roller bearing does not have mounting threaded holes, other members, such as a housing, a retainer plate, etc., are required for mounting the cross roller bearing, resulting in complication of mounting work. The above-mentioned rolling bearing has mounting threaded holes formed in the outer ring and the inner ring; however, since the threaded holes of the outer ring and the threaded holes of the inner ring are disposed on the radially outer and inner sides, respectively, of the rolling bearing, the thickness of the rolling bearing in the radial direction increases by an amount corresponding to the size of the threaded holes. In the case where the outer ring and the inner ring do not have the threaded holes, separate holders are used for mounting, resulting in complication of mounting work. Also, since the threaded holes of the holders are disposed on the radially outer and inner sides, respectively, of the rolling bearing, the overall radial thickness increases. In the above-mentioned cross roller bearing, the pieces of the outer ring have respective flanges formed on their outer circumferential surfaces, and screws are screwed into threaded holes of the flanges so as to fasten the pieces of the outer ring together. Accordingly, the cross roller bearing increases in thickness by the flanges. Therefore, for such a cross roller bearing, it is necessary to devise the structures of an outer ring and an inner ring, which structures allow the outer ring and the inner ring to have respective mounting portions and decrease the thickness of the cross roller bearing in the radial direction; in other words, to provide mounting holes for mounting mating members in the outer ring and the inner ring, respectively, without need to change the total radial thickness of the outer ring and the inner ring from the conventional radial thickness.